In the past few years, many micro-mechanical and micro-electromechanical devices (hereinafter collectively referred to as "MEMS devices") that include mechanical members have been fabricated from silicon or other etchable materials. These MEMS devices are advantageous because they can be made with microfabrication techniques having increased precision, allow for smaller miniaturization, and are generally lighter in weight.
The proliferation of MEMS devices having members comprised of silicon or other etchable materials has been mainly facilitated by the development of microfabrication techniques for the manufacture of integrated circuit chips. Specifically, the use of thin film processes has allowed the production of MEMS devices with submicron dimensional control. For example, micro-machines such as solid state laser and fiber optic couplings, ink jet nozzles and charge plates, gyroscopes and rotating plates, magnetic disks read/write heads, and optical recording heads can now be manufactured using silicon or other etchable materials. This use of etchable materials in manufacturing has allowed these micro-machines to be made smaller and more lightweight and with greater precision.
Although the production of MEMS devices having etched mechanical members has been expanding, several manufacturing problems have not yet been adequately addressed. For instance, unlike metallic materials, silicon and other etchable materials are generally more fragile. This characteristic of silicon and other etchable materials makes them more susceptible to fracture, especially during the manufacturing process. The delicate nature of silicon and other etchable materials is exacerbated by the fact that some MEMS devices include bladders or membranes or other mechanical members that are generally very thin. The thinness of these mechanical members coupled with the fragility of the etchable material from which they are made makes these mechanical members susceptible to fracture during the manufacturing process.
In many applications, significant processing is performed on the mechanical members. For instance, in some applications, conductive contact pads or traces are formed on the various surfaces of the micro-electronic substrate including the surface of any membrane or other mechanical member. The forming of these conductive pads or traces can subject the thin bladders, membranes or other mechanical members of the micro-electronic substrate to significant forces which may sometimes damage the membrane or other mechanical members. As such, a method is needed for manufacturing a MEMS device that includes mechanical members made of silicon or other etchable materials, which will allow for subsequent processing of the MEMS device without fear of fracturing or otherwise damaging the mechanical members.
Current techniques for manufacturing MEMS devices from silicon and other etchable materials also suffer from other problems. In this regard, many microfabrication techniques, such as photolithography, generally require a planar surface. With many current procedures for forming MEMS devices, however, a planar surface is difficult to maintain.
For instance, many etching procedures initially apply a layer of photoresist to the surface to be etched. The photoresist layer is then covered by a mask that defines regions of the photoresist that are to be exposed to light. Because light is used to expose the photoresist, it is important that the mask is in close contact with the photoresist to ensure that the pattern defined by the mask is precisely replicated upon exposure of the photoresist to light. If the photoresist layer is nonplanar, however, the mask may be have to be spaced from the photoresist layer, thereby affecting the precision to which the photoresist layer is illuminated and, in turn, developed.
One example of this problem occurs where the photoresist has been applied by use of a spinning procedure. This spinning procedure is usually accomplished by applying a desired amount of flowable photoresist on the surface of a substrate. The substrate is then rotated about an axis perpendicular to the surface of the substrate. During rotation, the flowable material is spread across the substrate by centrifugal force, and the surface of the substrate is covered with a layer of photoresist.
Although this procedure deposits a layer of photoresist on the micro-electronic substrate, the outer edges of the photoresist layer tend to define a ridge (referred to as an edge bead) due to the centrifugal force and the surface tension of the photoresist. This ridge has a greater thickness than the inner portions of the photoresist and can affect the application of the mask to the photoresist, and thereby, affect the precision of the subsequent etching procedures.
As stated previously, precise etching procedures are required for producing MEMS devices from silicon and other etchable materials. As such, a method of applying layers of material, such as photoresist, is needed that prevents the formation of ridges or edge beads on the outer edges of the material layer such that precision etching may thereafter be performed.